The ruling on the field was a fumble
The video evidence that the refs had painted a pretty strong picture that his knee was likely down...but maybe not absolutely irrefutable evidence given that some player's foot was between his knee and the camera, and the crush of bodies making it difficult to tell exactly when the ball started coming loose.
That's another case where I'd say that there was strong evidence that the call on the field was wrong...but maybe not absolutely concrete ironclad. So they made the wrong (meaning incorrect) call based off it being the initial call.
Bottom line, as I've gotten older I've become more OK with them using replay to get the best call...even if it's not 100% in the replay either